The present invention relates to a mobile communication system for carrying out radio communications with personal stations, and relates more particularly to a selection of a cell to be formed by a base station when radio communications are carried out between a mobile terminal station and a base station in a radio communication system of a multi-layer cell structure.
At present, in a land mobile communication system, there exist two kinds of size of a cell to be formed by a base station; a macro cell in a cellular car telephone system and a micro cell in a personal handy phone system. Further, there is a trend that a service area is going to be structured with pico cells for servicing indoors and at underground shopping centers. In the above-described systems, a cell selection is being implemented based on the following standard.
(1) When wave receiving intensity is deteriorated or when a fading generated is large due to a geographical condition of buildings, a cell of satisfactory circuit quality is selected at the time of a circuit connection, to thereby prevent a deterioration of service quality.
(2) In the area where a traffic density is high and a large number of channels are required such as in the center of the town, a cell of a smaller radius which has a potential of an increase in the system capacity is selected, to thereby cover a large capacity.
When one terminal is going to carry out communications with a plurality of mobile terminals in a mobile radio communication system of the structure as described above, in the state that the mobile terminals to be communicated with are widely distributed, there arise the following problems at the time of carrying out the communications through a cell that is determined based on the above-described standard.
(1) There is a possibility that a plurality of cells are required in order to cover all the mobile terminals to be communicated with that are distributed over a wide range. Particularly, when a distance between the mobile terminals to be communicated with is large, one cell is required for each one mobile terminal to be communicated with, so that it becomes necessary to carry out sequential communications with base stations by the number of the mobile terminals to be communicated with, which results in a long communication time.
(2) There is a risk of an occurrence of a capacity shortage of a cell when the cell covers an area of high distribution density with a large variation of distribution density within the same distribution range, such as, for example, when a town center is included in the distribution range of the plurality of terminals to be communicated with.
(3) Further, since the distribution density is different for each cell, there arises an imbalance in the processing load applied to each base station and each exchange respectively. A base station and an exchange applied with heavier processing load become a bottleneck because of this imbalance, with a result that the communication time becomes longer than expected and that this gives a large influence to other communication services such as, for example, an increase in the probability of call loss and an increase in the processing time.